1. Field of the Invention
This invention relates to a discharge lamp and, more particularly, to an igniter circuit which minimizes or substantially eliminates magnetostrictive-generated audio frequencies within the igniter during operation of the discharge lamp.
2. Description of the Related Art
The structure and operation of a discharge lamp, henceforth referred to as a "lamp" is generally well known. A lamp typically comprises a quartz tube filled with gas. The gas ambient is exposed to a pair of electrodes. During times when current is passed between the electrode pair, the gas is excited to a plasma state which causes photon emissions. Plasma excitation causes high intensity light emission from the lamp.
Lamps of this type are regulated by a ballast. Among other functions, a ballast provides current regulation across the electrode pair under conditions of changing voltage. Generally speaking, there are two types of ballast: an electronic ballast or a core-coil ballast. Regardless of its form, most ballast operably limit current to the lamp. Output from the ballast can be modeled as a regulated current source. Absent a ballast, rapid increases in voltage across the electrode pair can result in malfunction or damage of the lamp. The ballast can be a part of or separate from the AC power supply.
Lamps of this type generally utilize an igniter in conjunction with the ballast. The igniter essentially provides high voltage, short duration pulses which assist in initiating lamp ionization or discharge. Thus, while the ballast converts the AC line voltage to the proper amplitude and impedance level, an igniter superimposes a voltage pulse on the ballast output. The igniter pulse ignites the lamp and, after ignition, the voltage across the lamp settles back to a steady state value lower than the ignition pulse value. An igniter typically comprises various circuits components arranged in series between the ballast and the lamp. Most igniters include a step-up transformer. The AC ballast output is operably forwarded to one terminal of the primary and secondary windings of that transformer. When the transformer produces a voltage spike across the electrode pair, lamp break-over (or ionization occurs). During initial ionization, current across the primary and secondary windings, and between the anode-cathode rapidly rises.
Most transformer windings encircle tubular bobbins (either split or singular bobbins). Most bobbins are then enclosed within a ferromagnetic core made of, e.g., iron, steel or nickel. The core can be formed from a series of laminations which are clamped to form a rigid assembly of, for example, nickel iron suspended in ceramic (ferrite). As current rapidly increases and decreases (i.e., transitions) through the primary and secondary windings, electromagnetic ("EM") field of substantial magnitude transitions around the core and expands/contracts synchronous with twice the AC input frequency. A problem associated with rapid current and EM fluctuations within a transformer is the mechanical stress placed on the bobbins relative to the core. Under conditions of quickly saturating current and then quickly removing the current, expansion/contraction on the mechanical components causes a phenomenon known as "magnitostriction" and/or "electrostrictions". Magnitostriction is the characteristic of mechanical components and materials of those components which cause them to expand or contract under the influence of a magnetic field. Electrostriction is the same characteristic as magnitostriction, however, under the influence of an electric field.
If the transformer components and materials are subjected to a sufficiently strong EM field, generally at a frequency less than 20 KHz, the transformer will produce an audible noise output discernible to the user as a vibration or hum. The vibration generally coincides with the AC power supply peaks, or roughly twice the AC input frequency attributed from the igniter. Unfortunately, the commutation frequency through the transformer occur near the center of the audio spectrum which tends to exacerbate noise received on audio equipment placed proximate to the lamp control circuitry. This vibration at approximately twice the AC frequency interferes with high fidelity sound equipment placed near or within the same room as the lamp circuitry. An improvement in conventional ballast and/or igniter design for certain lamp applications is therefore needed.